1. Field of the Invention
The present invention generally relates to a method for detecting an end-point of a chemical mechanical polishing (hereinafter, referred to as “CMP”) process of a semiconductor device, and more specifically, to a method for detecting an end-point of a CMP process of a semiconductor device wherein when all polishing processes are performed using a nitride film as a polishing barrier film, a buffer layer including nitrogen is formed on the nitride film and a polishing process is performed so that the nitride film may be polished to a desired target without damage of the nitride film.
2. Description of the Prior Art
In general, for fineness, high capacity and high integration of a semiconductor device, a method for forming landing plug contact (hereinafter, referred to as “LPC”) is required so that multi-layered lines may be formed to connect each device electrically.
That is, when a transistor, a bit line and a capacitor are formed in a semiconductor device, a gate is formed on a semiconductor substrate, an insulating film is formed on the whole surface of the resulting structure. Then, the insulating film is planarized by a polishing process, and then a LPC mask (M/K) process and an etching process are performed.
Here, in the polishing process for planarizing the insulating film, the insulating film is completely removed until the upper portion of the gate electrode is exposed in order to prevent generation of step difference between cell and peri regions. Then, an insulating film is re-deposited on the resulting structure and a subsequent process is performed.
However, in the conventional polishing process, in order to completely remove the insulating film, the insulating film is polished for a predetermined time, and its thickness is measured to find that the insulating film is polished to a desired target. Otherwise, whether the hard mask nitride film of the gate electrode is exposed is required to be observed through a CD SEM. When a desired critical dimension (hereinafter, referred to as “CD”) of the gate hard mask is not secured by the polishing process, a polishing process is repeated or run is required to be scrapped. As a result, the process time is delayed and yield is degraded.
In order to solve the above-described problems, a method for detecting an end-point of an interlayer insulating film on real-time in a polishing process using an optical method or a motor current method has been currently developed. Of the above-described methods, a method for setting an end-point by measuring ammonia gas generated from when a nitride film is polished has been widely used because the method is rapid and precise.
In the above-described method, when films including nitrogen such as SiN or TiN are polished, ammonia (NH3) gas generated from a slurry is collected in an End Point Detector (EPD) as a gas state, thereby setting an end-point.
According to the principle, if the ammonia gas is converted into NO gas by heat action and enzyme action, NO gas is combined with ozone (O3) as shown in the following equation 1 to form NO2 and excited NO2*. When the optical chemical reaction generates light and NO gas is converted into NO2, the amount of NO can be measured by detecting the generated light.

FIGS. 1a to 1d are diagrams illustrating a conventional method for detecting an end-point of a CMP process.
Referring to FIG. 1a, a polysilicon layer 3, a conductive layer for gate electrode 5 and a hard mask nitride film 7 are sequentially formed on a semiconductor substrate 1 comprising a device isolation film (not shown).
A selective etching process is performed on the hard mask nitride film 7, the conductive layer for gate electrode 5 and the polysilicon layer 3 which are formed in FIG. 1a to form a gate line 8 having a sequentially stacked structure of a polysilicon pattern 3-1, a conductive pattern for gate electrode 5-1 and a hard mask nitride film pattern 7-1.
As shown in FIG. 1b, an oxide film (not shown) is formed on the whole surface of the resulting structure including the gate line 8.
As shown in FIG. 1c, an interlayer insulating film 13 is formed on the whole surface of the resulting structure including the gate line 8 and the nitride film spacer 10 which are formed in FIG. 1b. 
As shown in FIG. 1d, the interlayer insulating film 13 of FIG. 1c is polished until the hard mask nitride film pattern 7-1 is exposed.
However, when the polishing process of FIG. 1d is performed, the hard mask nitride film which is a polishing barrier film starts to be exposed, and the amount of NO gas increase because the amount of NH3 gas remarkably increases as shown in FIG. 1e. 
As a result, time for setting an end-point by detecting the concentration of NO runs short. Therefore, since an end-point is set after a predetermined thickness of the hard mask nitride film is polished, the hard mask nitride film is deteriorated and a stable subsequent process cannot be performed.